The new Fel Core Hound mount is still using the default particle colours for the model, orange-red, which were designed for the original red core hound and look a bit strange on this one. Traditionally green core hounds had green fire.

The model supports particle colour customization, so this could very easily be done for this mount. It only requires a new entry in ParticleColor.db2, and tweaking of the entry in CreatureDisplayInfo.db2 to reference it.

It could very easily just use the same particle colours as the original green core hound from Shadowmoon (Outland), the values of which are (red, green, blue):start: (58 , 208 , 24), middle: (100 , 255 , 24), end: (191 , 255 , 19)Or in #RRGGBB hex notation:start: #3AD018, middle: #64FF18, end: #BFFF13

I can show you the difference in WoW Model Viewer. While WMV doesn't render all the effects right now it can show you the difference in fire.

This is the mount with the current (default) fire colour: http://forums.wow-petopia.com/download/file.php?id=26618

And this is with the particle colours altered to be the same as the old green core hound, using the values I gave above: http://forums.wow-petopia.com/download/file.php?id=26619